The long-range objective of this project is to understand the mechanism used by bacterial cells to place the division site at its proper location at midcell. To approach this objective, the bacterial minicell locus (minB) will be studies since we have previously shown that the three min gene products play an important role during the normal cell cycle to ensure the proper placement of the division septum at midcell. This is accomplished by the combined action of a division inhibitor, dependent on the presence of the MinC and MinD proteins, and a topological specificity factor, the MinE protein. A combination of genetics, immunochemistry and immunoelectron microscopy, and biochemistry will be used, with the following goals: (1) to determine the cellular localization of the three Min proteins under normal conditions and under conditions where the topological specificity of septal placement is perturbed; (2) to determine which proteins interact with the Min proteins within the cell; (3) to identify the domain within MinE that is responsible for its ability to give topological specificity to the site-selection process, and to use genetic means to identify cellular proteins that play a role in the topological specificity function of MinE; (4) to use genetic means to identify cellular proteins that play a role in the MinC-mediated septation block; (5) to determine how the ATPase activity of MinC- mediated septation block; (5) to determine how the ATPase activity of MinD is coupled to its role in the site-selection process.